test(patterndetector): key fixture polarity off golden.json (#497)

[trilamsr]

Closes #497.

Root cause

TestPatternDetector_NegativeFixturesEmitNoVerdicts enumerated every
non-canonical subdir under module/pkg/replay/pod_evicted/ as a
negative fixture (filter: if f.Name == \"canonical\" { continue }).
PR #484 introduced _real_world/ as a slot for anonymized,
operator-shaped positive captures (see
module/pkg/replay/pod_evicted/_real_world/README.md — "contributed
fixtures land golden verdicts alongside"). The first such fixture,
synthetic-2026-06-multi-rank-disk-pressure/, ships a 3-verdict
golden.json (2 full + 1 partial). The detector was correct; the
test's negative-set definition was wrong.

Two symptom-only fixes were possible (skip _real_world/* by path,
or add a manifest-level positive: bool), but both leave the inverse
drift hole: a positive fixture demoted to silently-skipped would
still pass.

Fix

Replace path-based filtering with golden-driven dispatch:

• A fixture's own golden.json declares its polarity — empty []
  means "detector must emit nothing", non-empty means "detector
  must emit exactly this".
• Negative test (NegativeFixturesEmitNoVerdicts): skips fixtures
  whose golden is non-empty; remaining set asserts zero verdicts
  (unchanged contract).
• New positive test (PositiveFixturesMatchGolden): non-canonical
  fixtures with non-empty goldens must round-trip to that exact
  verdict slice. Closes the inverse-drift hole.

Self-aligning: future contributions under any group land in the
correct lane based on what their golden declares, not which subdir
they live in.

Verification

• cd module && GOWORK=off go test -run 'TestPatternDetector_NegativeFixturesEmitNoVerdicts|TestPatternDetector_PositiveFixturesMatchGolden' -v ./processor/patterndetectorprocessor/ — green (3 negatives + 1 positive subtest).
• cd module && GOWORK=off go test ./... — green, no new failures.
• make lint — 0 issues.
• Pre-push hooks (go vet, go mod verify, attribute-namespace-check, no-autoupdate-check) — green.

Test plan

• Reproduced RED on origin/main before edit.
• Both new test bodies run green locally.
• Full module test suite green.
• make lint green.

fix(test): pattern-detector replay test now keys fixture polarity off `golden.json` content rather than directory name, so positive `_real_world/` captures land in the positive-assertion lane and operator contributions plug in without test-code edits.